1. Field of the Invention
This invention relates to the manufacture of Portland cement clinker by calcining prior to the traditional rotary kiln.
2. Description of the Prior Art
In the manufacture of Portland cement clinker it is well known to practice calcination prior to clinkering in the rotary kiln. Energy to provide the heat in the calcining step is supplied by burners or feeders supplying fuel directly to the calcining vessel. These external fuel burners and fuel feeders are a source of mechanical and processing problems that lead to the instability of the system. Furthermore, the burners or fuel feeders, their storage, pumping and transport systems are expensive to build and maintain. Additionally, many of the lower cost waste fuels are not suitable for use because of flow, ignition and other like problems.
Garrett et al, U.S. Pat. No. 4,022,629, incorporated herein by reference discloses a method of manufacturing cement clinker wherein carbonaceous material is added to the cement raw material to provide a source of heat in the calcination stage. However, the calcination stage in Garrett et al is in the rotary kiln. Therefore, when using certain low-grade carbonaceous fuels, there are still serious combustion problems and lack of uniformity of heating in the calcination zone. While it is mentioned at column 6 of Garrett et al that the fuel may be added during the grinding operation, there is no recognition of any particular advantage in this. Furthermore, the indicated manner of introducing the fuel when preheater cyclones are employed is between the lower cyclones or between the lowest cyclone and the rotary kiln. This would result in the aforementioned problems inherent in fuel burners and fuel feeders.